def isPrime (a):
    if==1: return False
    test=True
    k=a-1
    while k>1:
        ifnot a%k:
            test=False
            break
        k-=1
        return test
n=input()
r=[]
for i in range(len(n)):
    for k in range (i+1,len(n)+1):
        a=int(n[i:k]
              if isPrime(a):
                  r+=[a]
print(max(r) if r else)
                  
